(1) Field of the Invention
This invention relates to a radiographic apparatus for picking up fluoroscopic images of an object under examination with radiation, and more particularly to a radiographic apparatus having a radiation grid for removing scattered radiation.
(2) Description of the Related Art
Medical institutions have radiographic apparatus installed therein for picking up fluoroscopic images of patients. As shown in FIG. 7, such a radiographic apparatus 51 includes a top board 52 for supporting a patient M, a radiation source 53 for emitting radiation, and a radiation detector 54 for detecting the radiation.
The radiation detector 54 has a radiation grid 55 placed on a radiation incidence plane thereof for removing scattered radiation produced from the patient M. As shown in FIG. 9A, the radiation grid 55 has elongated strips of absorbing foil 55a arranged as in a blind (see Japanese Unexamined Patent Publication No. 2002-257939, for example).
As shown in FIG. 9B, the radiation detector 54 has radiation detecting elements 54a arranged in a two-dimensional matrix form. Falling on this radiation detector 54 are a fluoroscopic image of the patient M as shown in FIG. 8A, and indirect radiation not removed by the radiation grid 55 and shadows of the radiation grid 55 as shown in FIG. 8B. In FIG. 8B, it appears as if the indirect radiation has uniform intensity. Actually, however, the indirect radiation presents a distribution corresponding to positions of the absorbing foil strips 55a. 
Components of this indirect radiation are obstructive to generation of the fluoroscopic image of the patient M. So, in recent years, a technique of acquiring fluoroscopic images of the patient M has been developed, which removes the components of indirect radiation from the pixels outputted from the radiation detector 54. When acquiring such a fluoroscopic image, the shadows of the absorbing foil strips 55a falling on the radiation detector 54 pose a problem. As shown in FIG. 9A, shadows S of the absorbing foil strips 55a fall on the radiation detector 54. When seen in plan, as shown in FIG. 9B, there appears a regular arrangement of lines of radiation detecting elements 54a including the shadows S of the absorbing foil strips 55a, and lines of radiation detecting elements 54a free of the shadows S of the absorbing foil strips 55a. 
The following method is used to remove components of indirect radiation from an image including the shadows S of the absorbing foil strips 55a. First, a region R of the image is assumed, which consists of a pixel including a shadow (dark pixel a2) and two pixels adjacent thereto and without shadows (light pixels a1 and a3). An equation is formed for each pixel of this region R, indicating that its pixel value is a sum of a component of direct radiation and a component of indirect radiation. Since the three pixels are included in the area R, simultaneous equations consisting of three equations are formed. That is, the following simultaneous equations are formed:Pixel value of dark pixel a2=component of direct radiation+component of indirect radiationPixel value of light pixel a1=component of direct radiation+component of indirect radiationPixel value of light pixel a3=component of direct radiation+component of indirect radiation
Components of direct radiation in the region R are obtained by solving these simultaneous equations. This operation may be carried out for the entire area of the image and the image reconstructed, thereby to acquire a fluoroscopic image without influences of the indirect radiation and well suited for diagnosis.
However, the conventional radiographic apparatus has the following drawback.
The conventional radiographic apparatus presupposes that positions of the shadows of the radiation grid 55 are always fixed relative to the radiation detector 54. In practice, however, the positions of the shadows can shift relative to the radiation detector 54 during radiography. In certain radiographic apparatus, the radiation source 53 and radiation detector 54 are revolvable about an axis extending longitudinally of the top board 2. In such radiographic apparatus, when fluoroscopic images are picked up while revolving the radiation source 53 and radiation detector 54, since the radiation source 53 and radiation detector 54 are heavy objects, those revolution distorts the structure supporting the radiation source 53 and radiation detector 54. This will shift a relative position between the radiation source 53 and radiation detector 54.
The above method of solving the simultaneous equations for the region R assumes that the shadow of absorbing foil strip 55a is not included in the light pixels. If the shadow of absorbing foil strip 55a moves to a position bridging the dark pixel and light pixel with revolution of the radiation source 53 and radiation detector 54, it will become impossible to separate the direct radiation from the indirect radiation accurately by the above method.